This invention relates to measurement of light lobs at a junction of two optical fibers, and is in particular concerned with testing the quality of splices in fiber communications systems.
At the present time, optical fibers are widely used for long-haul communications systems. In any given system, a long-distance fiber optic cable can consist of a number of fibers, each formed of several lengths of fiber joined end to end. In a given fiber there can be a number of splices separated from one another by a kilometer or more. Light is injected into each fiber at one end and a detector at the other end converts the information carried on the light into an electrical signal containing one or more channels. Each splice is a potential site of signal loss, and so the quality of the splices limits the quality and the distance for the communications system.
Splices can be fusion splices or mechanical splices. In a fusion splice the mating ends of the two fibers are cleaved and prepared so that their end surfaces are as square as possible. The fiber ends are manipulated on a jig so that the fibers are aligned on their x, y and z axes, and then the fibers are heat softened and fused together. The quality of the splice (i.e., the number of dB of loss) can be estimated by visual inspection of the fiber cores after fusion.
For a mechanical splice, the ends of the fibers are prepared, and are positioned in a V-groove in a splice coupler device. A matching oil (i.e. a clear fluid medium having the same retractive index as the optical fiber core) is applied at the junction of the fibers, and a cover or clamp is installed to hold the fibers securely in place. In this case, the positions of the fiber ends cannot be checked by visual inspection, so the amount of signal loss at the splices is inspected afterwards by analyzing light injected at one end of the fiber. This technique typically requires use of an optical time domain reflectometer (OTDR). This device is typically located at a site far from the location of the splice. Also, the OTDR is a rather expensive device, and requires considerable training for the operator. There have been several techniques proposed to inject light into the fiber on one side of a splice and extract light from the fiber on the other side of the splice, and from the measured light intensities compute the light loss at the splice.
A macrobending technique involves bending the fiber on each side of the splice, with each bend having a radius of curvature of 1 to 4 mm. Light is coupled from a focussed laser or LED source into a spot on the outside of the bend. With injectors of this type, -50 to -70 dBm of injection can be achieved, depending on the bend radius and light wavelength chosen. However, bending the fiber stresses it, typically at several times its proof stress limit, and can seriously compromise the long-term strength of the glass fiber. Tests have shown that fibers break in less than five minutes if the bend has a 1 mm radius. Consequently, macrobend light launchers employ bend radii of 4 mm or larger, which can achieve coupling factors of only -60 to -70 dBm at 1550 nm.
Microbend injectors do not impose such stress on the fibers, and thus impose less risk of fiber breakage. These devices typically distort the fiber by only about 0.1 micron along the fiber axis, and yet achieve a typical light launch efficiency of about -35 to 40 dBm at 1300 nm.
Microbend injectors bend the fiber over a small angle at one point, and impose a very small lateral distortion. These devices are tuned, that is, they are effective only at or near one given wavelength. If it is desired to inject light at another wavelength, the injector has to be entirely reconfigured for that wavelength.
In an evanescent microbend technique, the fiber is bent and is simultaneously impressed against the hypotenuse of a right-angle glass prism to cause microbending. This, in theory, provides about a 10 dB improvement over the macrobend injectors mentioned earlier. As tested, this type of device achieves an injection efficiency of -48 dBm at 1550 nm and -57 dBm at 1300 nm. This technique could also cause high stress.
Some previously proposed techniques as found in the patent literature include a macrobend technique as described in U.S. Pat. No. 4,618,212 (Ludington et al.) and a microbend technique as described in U.S. Pat. No. 4,652,123 (Neumann).
It has been desired to use a light injection system and light extraction system which have better efficiencies than those described above, and if possible at least -30 dBm, and preferably -20 dBM. However, the existing techniques could not reliably achieve these levels, thus limiting the ability to construct and use a field-developed splice tester for use in testing mechanical type splices.